Antenna directivity control method and radio apparatus

ABSTRACT

A connection request signal issued from a terminal (PS 2 ) corresponding to a new call and received by an array antenna ( 2 ) is supplied to an adaptive array ( 14 ) and a parameter estimating unit ( 15 ). The parameter estimating unit ( 15 ) lowers a send power of a terminal (PS 1 ) corresponding to an existing call by a predetermined level when it is detected from an output of the adaptive array ( 14 ) and a signal supplied from the antenna ( 2 ) that the connection request signal is received via a C-channel.

DESCRIPTION

[0001] 1. Technical Field

[0002] The present invention relates to a method of controlling antennadirectivity in a Spacial Division Multiple Access (SDMA) communicationsystem as well as a radio apparatus for a base station using the same.

[0003] 2. Background Art

[0004] A communication system, which uses a PHS (Personal Handy-phoneSystem) and has been widely used in recent years, employs a TDMA (TimeDivision Multiple Access) system, in which a frame of 5 ms(milliseconds) formed of four slots (1 slot=625 μs) is used as a basicunit for sending and receiving. The communication system using the PHSis already standardized as a “Second-Generation Cordless TelephoneConversation System”.

[0005] In this PHS, processing of measuring interference waves orundesired wave (U-waves) is performed during a control procedure forestablishing synchronization. The measurement of undesired waves isspecifically disclosed in “2nd-Generation Cordless TelephoneConversation System Standards RCR STR-28” (issued by Association ofRadio Industries and Businesses) specifying the standards of the PHS.

[0006]FIG. 12 illustrates a sequence flow of such processing ofmeasuring U-waves. Referring to FIG. 12, brief description will now begiven on such processing.

[0007] First, a PHS terminal sends a link channel establishment requestsignal to a base station via a C-channel. The PHS base station detectsan unused channel (unused T-channel) (i.e., performs carrier sensing),and sends a link channel allocation signal (LCH allocation signal)designating the unused T-channel to the PHS terminal side via theC-channel.

[0008] On the PHS terminal side, the U-wave measurement is performedbased on the link channel information received from the PHS base stationby determining whether the designated T-channel receives an interferencewave signal of a predetermined power or higher, or not. When theinterference wave signal of a predetermined power or higher is notdetected, i.e., when another PHS base station is not using thisdesignated T-channel, the synchronous burst signal is sent to the basestation via the designated T-channel, and the synchronization isestablished.

[0009] When the interference wave signal of a predetermined power orhigher is detected on the designated T-channel, i.e., when another PHSbase station is using the designated T-channel, the PHS terminal willrepeat the control procedure starting from sending of the signalrequesting the link channel establishment.

[0010] In the conventional PHS, a connection is achieved between theterminal and the base station via a communication channel, which cansuppress an interference wave and can achieve good communicationcharacteristics.

[0011] In recent years, another system using the SDMA communicationtechnology has been proposed for providing signals with reduced noisesand waveform distortion and improving a utilization efficiency of thechannel by allocating a single channel to multiple users in the samecell.

[0012] In the SDMA communication system, an adaptive array using anarray antenna is used as a base station. Operation principles of theadaptive array radio base station are disclosed, e.g., in the followingreferences.

[0013] B. Widrow, et al.: “Adaptive Antenna Systems”, Proc. IEEE, Vol.55, No. 12, pp. 2143-2159 (December 1967).

[0014] S. P. Applebaum: “Adaptive Arrays”, IEEE Trans. Antennas &Propag., Vol. AP-24, No. 5, pp. 585-598 (September 1976).

[0015] D. L. Frost, III: “Adaptive Least Squares Optimization Subject toLinear Equality Constraints,” SEL-70-055, Technical Report, No. 6796-2,Information System Lab., Stanford Univ. (August 1970).

[0016] B. Widow and S. D. Stearns: “Adaptive Signal Processing”,Prentice-Hall, Englewood Cliffs (1985).

[0017] R. A. Monzingo and T. W. Miller: “Introduction to AdaptiveArrays”, John Wiley & Sons, New York (1980).

[0018] J. E. Hudson: “Adaptive Array Principles”, Peter Peregrinus Ltd.,London (1981).

[0019] R. T. Compton Jr.: “Adaptive Antennas—Concepts and Performance”,Prentice-Hall Englewood Cliffs (1988).

[0020] E. Nicolau and D. Zaharia: “Adaptive Arrays”, Elsevier, Amsterdam(1989).

[0021] The radio wave, which is sent and received by the adaptive array,has a directivity in a predetermined direction, and the directivity ofthe radio wave area can be directed toward a mobile terminal.

[0022] In the SDMA system, it is possible to allocate the same channelto a plurality of users in the same cell as already described, and thecontents of this technique are specifically disclosed in “Blocking RatePerformance of SDMA with a 3-element Adaptive Array” by Daisuke TANAKAet al. (Technical Report of IEICE A. pp. 97-214, RCS97-252, MW97-197(1998-02)) issued by the Institute of Electronics, Information andCommunication Engineers, and others. The SDMA system is also referred asa PDMA (Path Division Multiple Access) system.

[0023]FIG. 13 conceptually shows terminals in communication with an SDMAbase station. As shown in FIG. 13, the SDMA communication technology isused in the PHS base station using the adaptive array so that oneSDMA-PHS base station 71 can accommodate a plurality of PHS terminalsPA-PF.

[0024] In FIG. 14, the adaptive array is used, and the directivity inonly one direction is conceptually represented. In this case, thedirectivity of the radio wave area can be set in a designated direction(main direction). At the same time, however, unnecessary radiation(directive radio wave area in a secondary direction) occurs from theradio wave area having the directivity in the designated direction. Forexample, if the directivity is formed as indicated by 73 a, unnecessaryradiation occurs in hatched regions 73 b and 73 c.

[0025] Accordingly, if a connection request for a new call is made inthe direction where the unnecessary radiation is present, thisunnecessary radiation is observed as an interference wave acting on thenew-call terminal, i.e., the terminal making the new call.

[0026] In an SDMA communication system, as disclosed in Japanese PatentLaying-Open No. 10-269603, when a multiplexer channel is to be allocatedto a terminal making a new call, a sending directivity for an existingcall is controlled to direct a null direction of the radio wave areatoward the new call so that the signal sent to the existing-callterminal, i.e., terminal making the existing call may not be determinedas an interference wave in the carrier sense performed by the new-callPHS terminal.

[0027] However, when the position of the new call is near the basestation, the level of radio wave radiated to a terminal PS2 making a newcall cannot be lowered to or below a specified level even if thedirectivity of the radio wave area of a terminal PS1 making an existingcall is controlled to direct its null direction toward new-call terminalPS2. This is because a depth of the null (i.e., an amount by which aradio wave intensity is suppressed in the null direction) is notpractically infinite.

[0028]FIG. 15 illustrates a sequence flow of control processing fordirecting the null direction of the radio wave area toward the new-callposition as described above. FIGS. 16A-16C conceptually show thedirectivity of the radio wave areas provided by a base station CS towardexisting-call PS1 in accordance with the above processing sequence.

[0029] Referring to FIGS. 15 and 16A-16C, it is first assumed thatexisting-call terminal PS1 and base station CS are in communication witheach other as shown in FIG. 16A.

[0030] Then, as shown in FIG. 16B, new-call terminal PS2 issues arequest for link channel establishment to base station CS whileexisting-call terminal PS1 and base station CS are in communication witheach other.

[0031] Referring to FIG. 15 again, when new-call terminal PS2 applies arequest for link channel establishment to base station CS, base stationCS sends a link channel allocation instruction to new-call terminal PS2.

[0032] New-call terminal PS2 performs carrier sense for measuring aninterference wave, and determines whether the channel, of whichallocation is instructed by base station CS, is a connectable channel ornot.

[0033] Base station CS keeps the communication with existing-callterminal PS1 and, at the same time, controls the null direction of theradio wave area to be directed toward new-call terminal PS2, as shown inFIG. 16C.

[0034] Referring to FIG. 15 again, it may be determined from a result ofthe carrier sense in new-call terminal PS2 that an interference wave hasonly a predetermined power or lower in the allocated channel. In thiscase, new-call terminal PS2 sends a synchronous burst signal to basestation CS.

[0035] In response to this, base station likewise sends a synchronousburst signal to new-call terminal PS2, and thereafter, synchronizationbetween base station CS and new-call terminal PS2 is established.

[0036] However, according to the control of the sending directivity ofbase station CS described above as well as the carrier sense operationin the new-call terminal, the depth of null is not infinite as describedabove when the position of the new-call terminal is close to basestation CS, even if the directivity of the radio wave area with respectto existing-call terminal PS1 is controlled to direct the null towardterminal PS2.

[0037] Therefore, such a case occurs that the radio wave level of theinterference wave in new-call terminal PS2 does not lower to or below aspecified value due to a radio wave emitted from base station CS. Inthis case, the channel is not allocated to the new-call terminal, whichcan originally communicate with the base station, so that the channelutilization efficiency cannot be improved.

DISCLOSURE OF THE INVENTION

[0038] An object of the invention is to provide an antenna directivitycontrol method, in which a directivity of a radio wave sent from an SDMAbase station can be controlled to improve a channel utilizing efficiencyof the SDMA base station, as well as an apparatus for the same.

[0039] For achieving the above object, a radio apparatus according toclaim 1 includes an array antenna provided with a plurality of antennas,and an adaptive array send control portion for controlling a send signalapplied to each of the antennas and performing spacial multiplexcommunication with a plurality of terminals. The adaptive array sendcontrol portion lowers an intensity of a radio wave emitted to a firstterminal for a predetermined period when a second terminal requests aconnection while communication with the first terminal is beingperformed.

[0040] According to claim 2, the radio apparatus according to the claim1 is further configured such that the adaptive array send controlportion lowers the intensity of the radio wave emitted to the firstterminal by a predetermined level for a predetermined period in responseto detection of the fact that a receiving level of a radio wave emittedfrom the second terminal is larger than a predetermined level when thesecond terminal requests the connection.

[0041] According to claim 3, the radio apparatus according to the claim1 is further configured such that the adaptive array send controlportion lowers the intensity of the radio wave emitted to the firstterminal by a level corresponding to a receiving level of a radio waveemitted from the second terminal for a predetermined period in responseto detection of the fact that the receiving level of radio wave emittedfrom the second terminal is larger than a predetermined level when thesecond terminal requests the connection.

[0042] According to claim 4, the radio apparatus according to the claim1 is further configured such that the adaptive array send controlportion lowers the intensity of the radio wave emitted to the firstterminal by a level corresponding to a receiving level of a radio waveemitted from the first terminal for a predetermined period when thesecond terminal requests the connection.

[0043] According to claim 5, the radio apparatus according to the claim1 is further configured such that the adaptive array send controlportion lowers the intensity of the radio wave emitted to the firstterminal by a level corresponding to a receiving level of a radio waveemitted from the first terminal for a predetermined period in responseto detection of the fact that a receiving level of a radio wave emittedfrom the second terminal is larger than a predetermined level when thesecond terminal requests the connection.

[0044] According to claim 6, the radio apparatus according to the claim1 is further configured such that the adaptive array send controlportion lowers the intensity of the radio wave emitted to the firstterminal by a level corresponding to receiving levels of radio wavesemitted from the first and second terminals for a predetermined periodin response to detection of the fact that the receiving level of radiowave emitted from the second terminal is larger than a predeterminedlevel when the second terminal requests the connection.

[0045] According to claim 7, the radio apparatus according to any one ofthe preceding claims 1 to 6 is further configured such that the adaptivearray send control portion operates to direct a null direction of aradio wave sent to the first terminal toward the second terminal whenthe second terminal requests the connection.

[0046] According to claim 8, an antenna directivity control methodincludes the steps of controlling a send signal to be applied to each ofa plurality of antennas of an array antenna, and establishing asend/receive channel in spacial multiplex communication with respect toa first terminal; lowering an intensity of a radio wave emitted to thefirst terminal for a predetermined period during measuring of aninterference wave by the second terminal when the second terminalrequests a connection while send/receive with respect to the firstterminal is being performed; and establishing the send/receive channelin spacial multiplex communication with respect to the second terminalby controlling a send signal to be applied to each of the antennas inaccordance with a result of the measurement of the interference wave bythe second terminal.

[0047] According to claim 9, the antenna directivity control methodaccording to the claim 8 is further configured such that the step oflowering the intensity of the radio wave emitted to the first terminalfor a predetermined period includes the step of lowering the intensityof the radio wave emitted to the first terminal by a predetermined levelin response to detection of the fact that a receiving level of a radiowave emitted from the second terminal is larger than a predeterminedlevel when the second terminal requests the connection.

[0048] According to claim 10, the antenna directivity control methodaccording to the claim 8 is further configured such that the step oflowering the intensity of the radio wave emitted to the first terminalfor a predetermined period includes the step of lowering the intensityof the radio wave emitted to the first terminal by a level correspondingto a receiving level of a radio wave emitted from the second terminal inresponse to detection of the fact that the receiving level of radio waveemitted from the second terminal is larger than a predetermined levelwhen the second terminal requests the connection.

[0049] According to claim 11, the antenna directivity control methodaccording to the claim 8 is further configured such that the step oflowering the intensity of the radio wave emitted to the first terminalfor a predetermined period includes the step of lowering the intensityof the radio wave emitted to the first terminal by a level correspondingto a receiving level of a radio wave emitted from the first terminalwhen the second terminal requests the connection.

[0050] According to claim 12, the antenna directivity control methodaccording to the claim 8 is further configured such that the step oflowering the intensity of the radio wave emitted to the first terminalfor a predetermined period includes the step of lowering the intensityof the radio wave emitted to the first terminal by a level correspondingto a receiving level of a radio wave emitted from the first terminal inresponse to detection of the fact that a receiving level of a radio waveemitted from the second terminal is larger than a predetermined levelwhen the second terminal requests the connection.

[0051] According to claim 13, the antenna directivity control methodaccording to the claim 8 is further configured such that the step oflowering the intensity of the radio wave emitted to the first terminalfor a predetermined period includes the step of lowering the intensityof the radio wave emitted to the first terminal by a level correspondingto receiving levels of radio waves emitted from the first and secondterminals in response to detection of the fact that the receiving levelof radio wave emitted from the second terminal is larger than apredetermined level when the second terminal requests the connection.

[0052] According to claim 14, the antenna directivity control methodaccording to any one of the preceding claims 8-13 is further configuredsuch that the step of lowering the intensity of the radio wave emittedto the first terminal for a predetermined period includes the step ofdirecting a null direction of the radio wave emitted to the firstterminal toward the second terminal when the second terminal requeststhe connection.

[0053] According to the invention, therefore, it is possible to lower alevel of an interference wave in the terminal requesting the connectionso that the connection can be easily established between the terminaland the apparatus provided with the antenna.

[0054] Further, according to the invention, it is possible to direct theantenna directivity toward a terminal other than the terminal, which isalready connected, while keeping a communication quality of thealready-connecetd terminal at a predetermined level or higher.Therefore, the former terminal in a different direction can be easilyconnected to the apparatus provided with the antenna without impedingthe communication with the already-connected terminal.

BRIEF DESCRIPTION OF THE DRAWINGS

[0055]FIG. 1 schematically shows a structure for performingcommunication between a PHS terminal PS2 and a PHS base station;

[0056]FIG. 2 is a schematic block diagram showing a structure of an SDMAbase station 1;

[0057] FIGS. 3A-3C illustrate a sequence flow of a channel allocatingoperation;

[0058]FIG. 4 conceptually illustrates spreading of a radio wave areafrom a base station CS in the case of channel allocation;

[0059] FIGS. 5A-5C conceptually show control for lowering a sendinglevel for existing call and directing a null direction of sendingdirectivity toward the existing call in multiplexer channel allocation;

[0060]FIG. 6 illustrates a sequence flow of an operation of an SDMA basestation 1 for performing channel allocation;

[0061]FIG. 7 is a flowchart illustrating processing of SDMA base station1 in a third embodiment;

[0062]FIG. 8 is a flowchart illustrating processing of controlling asending power in a fourth embodiment;

[0063]FIG. 9 is a flowchart illustrating processing of controlling asending power in a fifth embodiment;

[0064]FIG. 10 is a flowchart illustrating processing of controlling asending power in a sixth embodiment;

[0065]FIG. 11 is a flowchart illustrating processing of controlling asending power in a seventh embodiment;

[0066]FIG. 12 illustrates a sequence flow of processing of measuring aU-wave;

[0067]FIG. 13 conceptually shows a terminal communicating with an SDMbase station;

[0068]FIG. 14 conceptually shows radio wave directivity provided when anSDMA-P base station 71 is transmitting data to or from a PHS terminalPS1;

[0069]FIG. 15 illustrates a flow of control sequence for directing anull direction of a radio wave area toward a new-call; and

[0070] FIGS. 16A-16C conceptually show directivity of a radio wave areaextending from a base station CS to existing-call PS1.

BEST MODE FOR CARRYING OUT THE INVENTION

[0071] Embodiments of the invention will now be described with referenceto the drawings.

[0072] [First Embodiment]

[0073]FIG. 1 is a schematic view showing a structure for performingcommunication between a PHS terminal PS2 and a PHS base station.

[0074] Referring to FIG. 1, an SDMA-PHS base station 1, which will bereferred to as an “SDMA base station” hereinafter, forms a directiveradio wave area 3 a in a direction (main direction) toward a PHSterminal PS1. In this station, a PHS terminal PS2 enters an unnecessaryradiation area 3 b or 3 c (i.e., directive radio wave area in asecondary direction), which occurs simultaneously with directive radiowave area 3 a, for starting PHS communication.

[0075]FIG. 2 is a schematic block diagram showing a structure of SDMAbase station 1.

[0076] Referring to FIG. 2, SDMA base station 1 includes an arrayantenna 2 formed of antennas #1, #2, #3 and #4 of n (four in thisembodiment) in number, a switch SW13 a for switching a transmission pathdepending on whether an input signal supplied from array antenna 2 is aC-channel signal or a T-channel signal, a switch 18 for receiving asignal from switch SW13 a when a T-channel signal is supplied from arrayantenna 2, and switching a signal path depending on a receive mode and asend mode, a signal merging unit 17 for merging the T-channel signalsent from switch 18 and training information in the receive mode, aswitch SW13 b, which receives and selects the signals sent from switchSW13 a and signal merging unit 17 for transmitting the signal sent fromswitch SW13 a when the C-channel signal is input from array antenna 2,and for transmitting the signal sent from signal merging unit 17 whenthe T-channel signal is input from array antenna 2, a memory 16 forstoring the foregoing training information, an adaptive array receivingportion 14 for extracting a signal, which is sent from the terminal,from the information of C-channel supplied from switch 13 b or theinformation supplied from signal merging unit 17, a parameter estimatingunit 15 for obtaining a response vector from the information ofC-channel and the information applied from adaptive array 14, and amemory 16 for temporarily storing information applied from parameterestimating unit 15.

[0077] SDMA base station 1 further includes an adaptive array sendingportion 20, which receives a send signal Stx(t) in the sending mode, andproduces signals to be applied to respective antennas #1-#4 of arrayantenna 2 in accordance with a wait vector sent from parameterestimating unit 16. As will be described later, adaptive array sendingportion 20 controls intensity and directivity of a radio wave emittedfrom array antenna 2 in accordance with data sent from parameterestimating unit 16.

[0078] Although not shown, an analog-digital converter is arrangedbetween array antenna 2 and switch SW13 a.

[0079] When PHS terminal PS2 is operated to start a call after enteringunnecessary radiation area 3 c extending from directive radio wave area3 a provided by SDMA base station 1, PHS terminal PS2 sends a requestsignal for link channel establishment (i.e., connection request signal)to SDMA base station 1 via the C-channel.

[0080] For receiving the C-channel signal by SDMA base station 1,switches SW13 a and 13 b are set to send the signal sent from arrayantenna 2 to adaptive array receiving portion 14 so that the C-channelsignal is supplied to adaptive array receiving portion 14 via arrayantenna 2.

[0081] Signal lines extending from array antenna 2 are provided for eachantenna. For the antennas of n in number, n signal lines are providedfor each of adaptive antenna receiving portion 14, parameter estimatingunit 15 and signal merging unit 17. In this embodiment, since arrayantennas 2 are four in number, four signal lines are provided for eachof them.

[0082] Assuming that the C-channel signal sent from PHS terminal PS2 isrepresented by Scz(t), a C-channel received signal Xc1(t) on firstantenna #1 can be expressed by the following formula:

Xc1(t)=a1×Sc2(t)+n1(t)

[0083] where a1 is a factor changing for improvement in real time. AC-channel received signal on second antenna #2 is expressed as follows:

Xc2(t)−a2 33 Sc2(t)+n2(t)

[0084] where a2 is likewise a factor changing in real time.

[0085] Likewise, a C-channel received signal Xcn(t) on antenna #n in annth position among the antennas of n in number is expressed as follows:

Xcn(t)=an×Scn(t)+nn(t)

[0086] where an is a factor changing in real time.

[0087] Factors a1, a2, a3, a4, . . . and an described above representthat differences in intensity and phase occur between signals receivedby antennas #1, #2, #3, . . . and #n forming array antenna 2 becausethese antennas are located at different positions with respect to theradio signal sent from PHS terminal PS2, respectively. In thisembodiment, each antenna is spaced by about one meter, i.e., a distancelonger by five times than a wavelength of the radio signal.

[0088] Since each PHS terminal PS is moving, these factors change inreal time.

[0089] The foregoing n1, n2, n3, . . . and nn represent noises generatedin the respective antennas and receiving circuits.

[0090] Signals Xc1, Xc2, Xc3 and Xc4 received by the respective antennasare supplied to adaptive array receiving portion 14, which obtains andoutputs signal Scz(t), i.e., the C-channel signal sent from PHS terminalPS2.

[0091] Signals Xc1, Xc2, Xc3 and Xc4 received on the antennas are alsosupplied to parameter estimating unit 15, and correlative values C1, C2,C3 and C4 of received signals Xc1, Xc2, Xc3 and Xc4 with respect to theoutput signal of adaptive array receiving portion 14 are calculated,respectively, so that the array response vectors of the respectiveantennas can be obtained by the following formulas: $\begin{matrix}{C_{1} = \quad {\frac{\sum\limits_{t = 1}^{T}\quad \{ {{a_{1}{S_{CZ}(t)} \times {S_{CZ}(t)}} + {{n_{1}(t)} \times {S_{CZ}(t)}}} \}}{T \times {X_{C\quad 1}} \times {{S_{CZ}(t)}}} = a_{1}}} \\{C_{2} = \quad {\frac{\sum\limits_{t = 1}^{T}\quad \{ {{a_{2}{S_{CZ}(t)} \times {S_{CZ}(t)}} + {{n_{2}(t)} \times {S_{CZ}(t)}}} \}}{T \times {X_{C\quad 2}} \times {{S_{CZ}(t)}}} = a_{2}}} \\{C_{3} = \quad {\frac{\sum\limits_{t = 1}^{T}\quad \{ {{a_{3}{S_{CZ}(t)} \times {S_{CZ}(t)}} + {{n_{3}(t)} \times {S_{CZ}(t)}}} \}}{T \times {X_{C\quad 3}} \times {{S_{CZ}(t)}}} = a_{3}}} \\{C_{4} = \quad {\frac{\sum\limits_{t = 1}^{T}\quad \{ {{a_{4}{S_{CZ}(t)} \times {S_{CZ}(t)}} + {{n_{4}(t)} \times {S_{CZ}(t)}}} \}}{T \times {X_{C\quad 4}} \times {{S_{CZ}(t)}}} = a_{4}}} \\{\vdots \quad \vdots \quad \vdots} \\{C_{n} = \quad {\frac{\sum\limits_{t = 1}^{T}\quad \{ {{a_{n}{S_{CZ}(t)} \times {S_{CZ}(t)}} + {{n_{n}(t)} \times {S_{CZ}(t)}}} \}}{T \times {X_{Cn}} \times {{S_{CZ}(t)}}} = a_{n}}}\end{matrix}$

[0092] Thereby, parameter estimating unit 15 obtains and outputs arrayresponse vectors a1, a2, a3 and a4 of the respective antennas.

[0093] Array response vectors a1, a2, a3 and a4 output from parameterestimating unit 15 are supplied to memory 16 for temporary storage.

[0094] Then, SDMA base station 1 sends a link channel allocation signal,which designates the T-channel allowing connection, via the C-channel toPHS terminal PS2, which sent the request signal for link channelestablishment via the C-channel.

[0095] In the above operation, it is assumed that SDMA base station 1designates the same T-channel and the same frequency as those used byPHS terminal PS1.

[0096] Based on the link channel information sent from SDMA base station1, PHS terminal PS2 measures a U-wave on the designated T-channel, andthus determines whether it has received an interference wave signal of apredetermined power or higher on the designated T-channel or not.

[0097] During this operation, the directive radio wave directed towardPHS terminal PS1 causes unnecessary radiation of a radio wave in aposition of PHS terminal PS2. Therefore, a signal of a power of apredetermined value or higher is detected on the designated T-channel sothat the synchronous burst signal cannot be sent.

[0098] In this state, PHS terminal PS1 is already connected to SDMA basestation 1, and is using the T-channel for communication.

[0099] The SDMA base station 1 of the first embodiment lowers a sendingpower for existing-call terminal PS1 at least while new-call terminalPS2 is performing carrier sense, as will be described below.

[0100] For example, the sending power is lowered by a level of 20 dB,although no restricted thereto.

[0101] Since the synchronous channel is established with respect toexisting-call terminal PS1 owing to the radio wave area, which isprovided by the adaptive antenna and has the directivity, the goodcommunication channel can be ensured even when the sending power islowered by the above level. In contrast to the above, new-call terminalPS2 is in such as state that terminal PS2 receives, as an interferencewave, an unnecessary radio wave radiated in a secondary direction otherthan the main direction of existing-call terminal PS1. Therefore, thelowering of the sending power by the above level causes such a statethat the above unnecessarily radiated radio wave takes a value enough tolower the level of the interference wave in the position of the new-callterminal PS2.

[0102] FIGS. 3A-3C illustrate a sequence flow of the channel allocationoperation. FIG. 4 conceptually illustrates spreading of the radio wavearea from base station CS during the channel allocation.

[0103] Referring to FIGS. 3A-3C and 4, it is first assumed that basestation CS and existing-call terminal PS1 are already in communicationwith each other as shown in FIG. 3A.

[0104] In this state, new-call terminal PS2 issues a link channelestablishment request via the C-channel to base station CS, as shown inFIG. 3B.

[0105] Referring to FIG. 4, base station CS instructs allocation of theT-channel for the instruction of link channel allocation in response tothe link channel establishment request issued from new-call terminal PS2to base station CS.

[0106] In response to the above, new-call terminal PS2 performs thecarrier sense for measuring the interference wave level (step S11). Inparallel with this, base station CS performs the processing for loweringthe sending power for the existing-call by a predetermined level (stepS12). This state is shown in FIG. 3C.

[0107] Since the unnecessary radiation level lowers in the position ofnew-call terminal PS2, the interference wave level lowers in theposition of new-call terminal PS2.

[0108] According to the above processing, as shown in FIG. 4, new-callterminal PS2 sends the synchronous burst signal to base station CS ifthe interference wave level in new-call terminal PS2 is at or lower thanthe predetermined level. In response to the sending of the synchronousburst signal, base station CS returns the synchronous burst signal tonew-call terminal PS2 so that the synchronized state is establishedbetween the base station and new-call terminal PS2.

[0109] Owing to the above operations, times of occurrence of such asituation are reduced that a result of the carrier sense by the new-callterminal does not satisfy the specifications after allocation of thechannel to the new-call terminal by the base station.

[0110] Thereby, the average time required before start of conversationcan be reduced. Further, it is possible to improve a call successprobability of new call to the base station.

[0111] [Second Embodiment]

[0112] According to the sending directivity control of SDMA base station1 of the first embodiment, when a multiplex channel is to be allocatedto a new-call in response to the link channel connection request issuedby the new-call, the sending power for the existing-call is lowered by apredetermined level for a period of the carrier sense performed by thenew-call terminal, and thereby the interference wave level in theposition of the new-call is lowered. According to a second embodiment,when SDMA base station 1 allocates a multiplex channel to a new-call, itlikewise lowers the sending power for the existing-call by apredetermined level during a period of the carrier sense performed bythe new-call. In addition to this, SDMA base station 1 controls thesending directivity for the existing-call so that the null direction ofthe sent radio wave area for the existing-call may be directed towardthe new-call.

[0113] FIGS. 5A-5C conceptually show the control performed in themultiplex channel allocation operation for lowering the sending levelfor the existing-call and directing the null direction of sendingdirectivity for the existing-call toward the existing-call. FIG. 6illustrates a sequence flow of the operation of SDMA base station 1 forperforming such channel allocation.

[0114] Referring to FIGS. 5A-5C and FIG. 6, it is first assumed thatbase station CS and existing-call terminal PS1 are already incommunication with each other.

[0115] In this state, it is assumed that new-call terminal PS2 issues alink channel establishment request to base station CS as shown in FIG.5B.

[0116] Referring to FIG. 6, base station CS instructs allocation of theT-channel for the instruction of link channel allocation in response tothe link channel establishment request issued from new-call terminal PS2to base station CS.

[0117] In response to the above, new-call terminal PS2 performs thecarrier sense for measuring the interference wave level (step S11). Inparallel with this, base station CS performs the processing fordirecting the null direction of the sending directivity toward theexisting-call and lowering the sending power for the existing-call by apredetermined level (step S12′). This state is shown in FIG. 5C.

[0118] Since the unnecessary radiation level lowers in the position ofnew-call terminal PS2, the interference wave level lowers in theposition of new-call terminal PS2.

[0119] According to the above processing, as shown in FIG. 6, new-callterminal PS2 sends the synchronous burst signal to base station CS ifthe interference wave level in new-call terminal PS2 is at or lower thanthe predetermined level. In response to the sending of the synchronousburst signal, base station CS returns the synchronous burst signal tonew-call terminal PS2 so that the synchronized state is establishedbetween base station CS and terminal PS2.

[0120] [Null Direction Control by SDMA Base Station]

[0121] As already described in connection with the first embodiment,when new-call terminal PS2 sends the link channel establishment requestvia the C-channel, base station CS obtains array response vectors a1,a2, a3 and a4 with respect to the C-channel signal sent from new-callterminal PS2. Description will now be given in greater detail on theabove operation as well as the operation of controlling for directingthe null direction of the sending directivity for the existing-calltoward new-call terminal PS2.

[0122] For receiving the C-channel signal by SDMA base station 1, switchSW13 a selects signal merging unit 17 so that the T-channel signal issupplied to signal merging unit 17 via array antenna 2.

[0123] Assuming that PHS terminal PS1 sends a T-channel signal STA(t), aT-channel received signal Xt1(t) on first array antenna #1 is expressedby the following formula:

Xt1(t)=b1×StA(t)+n1(t)

[0124] where b1 is a factor changing for improvement in real time.

[0125] Likewise, if there are n antennas, T-channel received signalXtn(t) on nth antenna #n can be expressed by the following formula:

Xtn(t)=bn×StA(t)+nn(t)

[0126] where bn is also a factor changing in real time.

[0127] Factors b1, b2, b3, b4, . . . and bn described above representthat differences in intensity and phase occur between signals receivedby antennas #1, #2, #3, . . . and #n forming array antenna 2 becausethese antennas are located at different positions with respect to theradio signal sent from PHS terminal PS1, respectively. Since each PHSterminal is moving, these factors change in real time. The foregoing n1,n2, n3, . . . and nn represent noises generated in the respectiveantennas and receiving circuits.

[0128] Signals Xt1, Xt2, Xt3 and Xt4 received by the respective antennasare supplied to signal merging unit 17. Signal merging unit 17 producesT-channel information, which is sent from PHS terminal PS2 falselydetermined by the respective antennas, from array response vectors a1,a2, a3 and a4 of PHS terminal PS2 on the respective antennas, which aretemporarily stored in memory 16, and a false T-channel signal ST2(t) ofPHS terminal PS2, which is already produced and stored in memory 16.

[0129] Signal merging unit 17 merges T-channel received signal Xt1, Xt2,Xt3 and Xt4 of PHS terminal PS1 received by antenna 2 with the foregoingT-channel information sent from the false PHS terminal PS2. Adaptivearray receiving portion 14 is supplied with merged signals Xt1′, Xt2′,Xt3′ and Xt4′ thus produced by signal merging unit 17 from signals ofPHS terminals PS1 and PS2 on the respective antennas. Memory 16 hasinternally prepared and stored in advance a T-channel signal ST2(t) ofPHS terminal PS2, which is estimated as the signal to be sent thereto.

[0130] Merged signal Xt1′(1) on the first antenna #1 is expressed by thefollowing formula:

Xt1′(t)=b1T×StA(t)+a1×STZ(t)+n1(t)

[0131] Likewise, if there are n antennas, merged signal Xtn′(t) on thenth antenna #n is expressed by the following formula:

Xtn′(t)=bnT×StA(t)+an×STZ(t)+nn(t)

[0132] Accordingly, merged signals Xt1′(t), Xt2′(t), . . . and Xt4′(t)supplied to adaptive array receiving portion 14 are merged signalsformed of (b1×StA(t), . . . , b4×StA(t)) supplied from PHS terminal PS1,signals (a1×StZ(t), . . . , a4×StZ(t)) supplied from PHS terminal PS2,and noises, respectively.

[0133] The above merged signal includes the signal sent from PHSterminal PS1, which is currently in communication via the T-channeldesignated by the PHS base station, and practically includes anadditional signal, which is provided by a radio wave sent from PHSterminal PS2 not actually sending a radio wave on the T-channel.

[0134] If the adaptive array antenna operates, e.g., in accordance withRLS (Recursive Least Square) algorithm, the directivity in the maindirection is directed toward the desired signal, and a null point isformed for the interference signal.

[0135] Accordingly, when adaptive array receiving signal 14 uses themerged signal of signals of PHS terminals PS1 and PS2 for controllingthe directivity, control can be performed as shown in FIGS. 5A-5C. Morespecifically, the directivity toward PHS terminal PS1 (in the maindirection) is adjusted to maintain the directive radio wave area so thatthe communication quality for PHS terminal PS1 is kept at apredetermined level or higher, and the sending power for PHS terminalPS1 is reduced by a predetermined value. Further, the null point of theantenna directivity is directed toward PHS terminal PS2 while givingconsideration also to unnecessary radiation (directive radio wave areain a secondary direction). Therefore, the interference wave level can bereduced in PHS terminal PS2.

[0136] Thereby, the radio wave sent from SDMA base station 1 is kept ator below a predetermined level in the position of PHS terminal PS2performing the U-wave measurement so that the U-wave measurement in PHSterminal PS2 can be normally completed, and it is possible to performthe processing in and after the step of sending of the synchronous burstsignal using the T-channel.

[0137] SDMA base station 1 receives the signal including the synchronousburst signal sent from PHS terminal PS2 via the T-channel, and controlsthe directivity to extract this synchronous burst signal so that adirective area 4 a can be formed in the direction toward PHS terminalPS2, and PHS terminal PS2 can communicate with SDMA base station 1 viathe T-channel.

[0138] [Third Embodiment]

[0139] In the first embodiment, when the SDMA base station allocates amultiplex channel to a new-call, the sending power for the existing-callis lowered by a predetermined level while the new-call terminal isperforming the carrier sense.

[0140] According to a third embodiment, however, when SDMA base station1 allocates the multiplex channel to the existing-call, step S12 of thefirst embodiment illustrated in FIG. 4 is not employed. Alternatively,the receiving level of radio wave emitted from the new-call is measured,and the sending power for the existing-call is lower by a predeterminedlevel during the carrier sense by the new-call, if the receiving levelthus measured is higher than a predetermined level.

[0141]FIG. 7 is a flowchart illustrating a flow of such processing inSDMA base station 1 of the third embodiment.

[0142] When the new-call requests the channel allocation (step S100),the SDMA base station measures the receiving level of radio wave emittedfrom the new-call, and substitutes the measured value in a variableRSSI_NEW (step S102).

[0143] Subsequently, SDMA base station 1 compares the value of variableRSSI_NEW with a predetermined value (step S104), and lowers the sendingpower for the existing-call by a predetermined level (step S106) if thereceiving level of radio wave emitted from the new-call is larger than apredetermined value.

[0144] From the comparison of the value of variable RSSI_NEW with thepredetermined value performed by SDMA base station 1 in step S104, itmay be determined that the receiving level of radio wave emitted fromthe new-call is not higher than the predetermined level. In this case,the processing ends without changing the sending power for theexisting-call (step S108).

[0145] In the third embodiment, therefore, the processing of loweringthe sending power for the existing-call is performed only when thenew-call is located relatively near SDMA base station 1.

[0146] The predetermined level, by which the sending power is lowered,can be equal, e.g., to 20 dB, similarly to the first embodiment.

[0147] If the new-call is sufficiently far from SDMA base station 1, andit can be considered that an interference wave is at a sufficiently lowlevel not requiring lowering of the sending power for the existing-call,it is possible by the above processing to eliminate the processing oflowering the sending power level for the existing-call so that anaverage time required before starting conversation can be furtherreduced.

[0148] [Modification of Third Embodiment]

[0149] In the third embodiment, when SDMA base station 1 allocates amultiplex channel to a new-call, the receiving level of radio waveemitted from the new-call is measured, and the sending power for theexisting-call is lowered by a predetermined level during the carriersense by the new-call terminal, if the measured level is higher than apredetermined level.

[0150] However, processing in step S12′ of the second embodiment shownin FIG. 6 may be eliminated, and alternatively, the receiving level ofradio wave emitted from the new-call may be measured, and control can beperformed as follows. If the receiving level thus measured is higherthan the predetermined level, the sending power for the existing-call islowered by a predetermined level, and further the sending directivityfor the existing-call is adjusted to direct the null direction of thesent radio wave area for the existing-call toward the new-call.

[0151] [Fourth Embodiment]

[0152] In SDMA base station 1 of a fourth embodiment, the processing instep S12 of the first embodiment shown in FIG. 4 is not performed, andalternatively, a multiplex channel is allocated to a new-call for thespacial multiplex communication in such a manner that the receivinglevel of radio wave emitted from the new-call is measured during thecarrier sense by the new-call terminal, and the sending power for theexisting-call is lowered in accordance with the receiving level of radiowave emitted from the new-call if the measured receiving level of radiowave emitted from the new-call is higher than a predetermined level.

[0153]FIG. 8 is a flowchart illustrating such sending power controlprocessing of the fourth embodiment.

[0154] Referring to FIG. 8, when the new-call requests the channelallocation (step S200), SDMA base station 1 measures the receiving levelof radio wave emitted from the new-call, and substitutes the measuredvalue in variable RSSI_NEW (step S202).

[0155] Subsequently, SDMA base station 1 compares the value of variableRSSI_NEW with a predetermined value (step S204), and performs a newprocessing step S206 for changing the sending power for theexisting-call if the receiving level of radio wave emitted from thenew-call is larger than the predetermined value.

[0156] Assuming that a power difference of D_Power (dB) is presentbetween up and down of the system, SDMA base station 1 calculates alevel P_Down (dB), by which the level is to be lowered, in accordancewith the following formula (step S206):

[0157] P_Down=D_Power+RSSI_NEW—44 . . . (1)

[0158] Level P_Down for lowering can be calculated by the followingformula:

[0159] P_Down=D_Power+RSSI_NEW—44+Margin . . . (1′)

[0160] SDMA base station 1 ends the processing without changing thesending power for the existing-call (step S208) if the receiving levelof radio wave emitted from the new-call is not higher than thepredetermined level as a result of the comparison of the value ofvariable RSSI_NEW with the predetermined value performed in step S204.

[0161] Power difference D_Power between up and down of the system isspecified depending on the system and is equal, e.g., to 17 dB.

[0162] The predetermined level is specified depending on the system andis equal to, e.g., 44 dBμV. A value of margin Margin is appropriatelydetermined depending on the system and is equal, e.g., to 10 dB.

[0163] Similarly to the second embodiment, it is possible in the fourthembodiment to add the processing of controlling the sending directivityof the existing-call to direct the null point toward the new-call.

[0164] [Fifth Embodiment]

[0165] In a fifth embodiment, when the multiplex channel is to beallocated to a new-call, SDMA base station 1 measures the receivinglevel of radio wave emitted from the existing-call while the new-call isperforming the carrier sense, and lowers the sending power for theexisting-call in accordance with the receiving level of radio waveemitted from the existing-call.

[0166]FIG. 9 is a flowchart illustrating such sending power controlprocessing of the fifth embodiment.

[0167] Referring to FIG. 9, when the new-call requests the channelallocation (step S300), SDMA base station 1 measures the receiving levelof radio wave emitted from existing-call, and substitutes the measuredvalue in variable RSSI_GIVEN (step S302). However, the receiving levelof radio wave emitted from the existing-call can be read from the memorybecause the receiving level of radio wave emitted from the existing-callis usually monitored and stored in the memory during the communicationbetween the existing-call and the base station 1.

[0168] Assuming that the power difference of D_Power (dB) is presentbetween up and down of the system, SDMA base station 1 calculates levelP_Down (dB), by which the level is to be lowered, in accordance with thefollowing formula (step S304):

[0169] P_Down=D_Power+RSSI_GIVEN—44 . . . (2)

[0170] Level P_Down for lowering can be calculated by the followingformula:

[0171] P_Down=D_Power+RSSI_GIVEN—44+Margin . . . (2′)

[0172] Power difference D_Power between up and down of the system isspecified depending on the system and is equal, e.g., to 17 dB.

[0173] The value of margin Margin is appropriately determined dependingon the system and is equal, e.g., to 10 dB.

[0174] Similarly to the second embodiment, it is possible in the fifthembodiment to add the processing of controlling the sending directivityof the existing-call to direct the null point toward the new-call.

[0175] [Sixth Embodiment]

[0176] In a sixth embodiment, when the multiplex channel is to beallocated to a new-call, SDMA base station 1 measures the receivinglevels of radio waves emitted from the new-call and existing-call whilethe new-call is performing the carrier sense, and lowers the sendingpower for the existing-call in accordance with the receiving level ofradio wave emitted from the existing-call if the receiving level ofradio wave emitted from the new-call is higher than a predeterminedlevel.

[0177]FIG. 10 is a flowchart illustrating such sending power controlprocessing of the sixth embodiment.

[0178] Referring to FIG. 10, when the new-call requests the channelallocation (step S400), SDMA base station 1 reads out the receivinglevel of radio wave, which is emitted from the existing-call, from thememory, and substitutes the read value in variable RSSI_GIVEN (stepS402).

[0179] Subsequently, SDMA base station 1 measures the receiving level ofradio wave emitted from the new-call, and substitutes the measured valuein variable RSSI_NEW (step S404).

[0180] Then, SDMA base station 1 compares the value of variable RSSI_NEWwith a predetermined value (step S406), and performs a new processingstep S408 for changing the sending power for the existing-call if thereceiving level of radio wave emitted from the new-call is larger thanthe predetermined value.

[0181] Assuming that the power difference of D_Power (dB) is presentbetween up and down of the system, SDMA base station 1 calculates levelP_Down (dB), by which the level is to be lowered, in accordance with thefollowing formula (step S408):

[0182] P_Down=D_Power+RSSI_GIVEN—44 . . . (3)

[0183] Level P_Down for lowering can be calculated by the followingformula:

[0184] P_Down=D_Power+RSSI_GIVEN—44+Margin . . . (3′)

[0185] SDMA base station 1 ends the processing without changing thesending power for the existing-call (step S410) if the receiving levelof radio wave emitted from the new-call is not higher than thepredetermined level as a result of the comparison of the value ofvariable RSSI_NEW with the predetermined value performed in step S406.

[0186] Power difference D_Power between up and down of the system isspecified depending on the system and is equal, e.g., to 17 dB.

[0187] The predetermined level is specified depending on the system andis equal to, e.g., 44 dBμV. The value of margin Margin is appropriatelydetermined depending on the system and is equal, e.g., to 10 dB.

[0188] Similarly to the second embodiment, it is possible in the sixthembodiment to add the processing of controlling the sending directivityof the existing-call to direct the null point toward the new-call.

[0189] [Seventh Embodiment]

[0190] In a seventh embodiment, when the multiplex channel is to beallocated to a new-call, SDMA base station 1 measures the receivinglevels of radio waves emitted from the new-call and existing-call whilethe new-call is performing the carrier sense, and lowers the sendingpower for the existing-call in accordance with the receiving levels ofradio waves emitted from the new-call and existing-call if the receivinglevels of radio wave emitted from the new-call and existing-call arehigher than predetermined levels, respectively.

[0191]FIG. 11 is a flowchart illustrating such sending power controlprocessing of the seventh embodiment.

[0192] Referring to FIG. 11, when the new-call requests the channelallocation (step S500), SDMA base station 1 reads out the receivinglevel of radio wave, which is emitted from existing-call, from thememory, and substitutes the read value in variable RSSI_GIVEN (stepS402).

[0193] Subsequently, SDMA base station 1 measures the receiving level ofradio wave emitted from new-call, and substitutes the measured value invariable RSSI_NEW (step S504).

[0194] Then, SDMA base station 1 compares the value of variable RSSI_NEWwith a predetermined value (step S506), and performs a new processingstep S508 for changing the sending power for the existing-call if thereceiving level of radio wave emitted from the new-call is larger thanthe predetermined value.

[0195] Thus, SDMA base station 1 calculates level P_Down (dB), by whichthe level is to be lowered, in accordance with the following formula(step S508) from receiving level RSSI_NEW of radio wave emitted from thenew-call, receiving level RSSI_GIVEN of radio wave emitted from theexisting-call and the power difference D_Power between up and down ofthe system:

[0196] P_Down=D_Power+RSSI_NEW—44+(Margin+(RSSI-NEW−RSSI_GIVEN)/□) . . .(4)

[0197] Power difference D_Power between up and down of the system isspecified depending on the system and is equal, e.g., to 17 dB. Thepredetermined level is specified depending on the system and is equalto, e.g., 44 dBμV.

[0198] The value of margin Margin is appropriately determined dependingon the system and is equal, e.g., to 5 dB. The factor □ can be equal,e.g., to 2.

[0199] Similarly to the second embodiment, it is possible in the seventhembodiment to add the processing of controlling the sending directivityof the existing-call to direct the null point toward the new-call.Thereby, an influence on the new-call by the interference wave can besuppressed more effectively.

[0200] Although the present invention has been described and illustratedin detail, it is clearly understood that the same is by way ofillustration and example only and is not to be taken by way oflimitation, the spirit and scope of the present invention being limitedonly by the terms of the appended claims.

1. A radio apparatus comprising: an array antenna (2) provided with aplurality of antennas; and an adaptive array send control portion (20)for controlling a send signal applied to each of said antennas andperforming spacial multiplex communication with a plurality ofterminals, wherein said adaptive array send control portion lowers anintensity of a radio wave emitted to a first terminal (PS1) for apredetermined period when a second terminal (PS2) requests a connectionwhile communication with said first terminal is being performed.
 2. Theradio apparatus according to the claim 1, wherein said adaptive arraysend control portion lowers the intensity of the radio wave emitted tosaid first terminal by a predetermined level for a predetermined periodin response to detection of the fact that a receiving level of a radiowave emitted from said second terminal is larger than a predeterminedlevel when said second terminal requests the connection.
 3. The radioapparatus according to the claim 1, wherein said adaptive array sendcontrol portion lowers the intensity of the radio wave emitted to saidfirst terminal by a level corresponding to a receiving level of a radiowave emitted from said second terminal for a predetermined period inresponse to detection of the fact that the receiving level of radio waveemitted from said second terminal is larger than a predetermined levelwhen said second terminal requests the connection.
 4. The radioapparatus according to the claim 1, wherein said adaptive array sendcontrol portion lowers the intensity of the radio wave emitted to saidfirst terminal by a level corresponding to a receiving level of a radiowave emitted from said first terminal for a predetermined period whensaid second terminal requests the connection.
 5. The radio apparatusaccording to the claim 1, wherein said adaptive array send controlportion lowers the intensity of the radio wave emitted to said firstterminal by a level corresponding to a receiving level of a radio waveemitted from said first terminal for a predetermined period in responseto detection of the fact that a receiving level of a radio wave emittedfrom said second terminal is larger than a predetermined level when saidsecond terminal requests the connection.
 6. The radio apparatusaccording to the claim 1, wherein said adaptive array send controlportion lowers the intensity of the radio wave emitted to said firstterminal by a level corresponding to receiving levels of radio wavesemitted from said first and second terminals for a predetermined periodin response to detection of the fact that the receiving level of radiowave emitted from said second terminal is larger than a predeterminedlevel when said second terminal requests the connection.
 7. The radioapparatus according to claim 1, wherein said adaptive array send controlportion operates to direct a null direction of a radio wave sent to saidfirst terminal toward said second terminal when said second terminalrequests the connection.
 8. An antenna directivity control methodcomprising the steps of: controlling a send signal to be applied to eachof a plurality of antennas of an array antenna, and establishing asend/receive channel in spacial multiplex communication with respect toa first terminal; lowering an intensity of a radio wave emitted to saidfirst terminal for a predetermined period during measuring of aninterference wave by said second terminal when said second terminalrequests a connection while send/receive with respect to said firstterminal is being performed (step S12); and establishing thesend/receive channel in spacial multiplex communication with respect tosaid second terminal by controlling a send signal to be applied to eachof said plurality of antennas in accordance with a result of themeasurement of the interference wave by said second terminal.
 9. Theantenna directivity control method according to the claim 8, whereinsaid step of lowering the intensity of the radio wave emitted to saidfirst terminal for a predetermined period includes the step (S106) oflowering the intensity of the radio wave emitted to said first terminalby a predetermined level in response to detection of the fact that areceiving level of a radio wave emitted from said second terminal islarger than a predetermined level when said second terminal requests theconnection.
 10. The antenna directivity control method according to theclaim 8, wherein said step of lowering the intensity of the radio waveemitted to said first terminal for a predetermined period includes thestep (S208) of lowering the intensity of the radio wave emitted to saidfirst terminal by a level corresponding to a receiving level of a radiowave emitted from said second terminal in response to detection of thefact that the receiving level of radio wave emitted from said secondterminal is larger than a predetermined level when said second terminalrequests the connection.
 11. The antenna directivity control methodaccording to the claim 8, wherein said step of lowering the intensity ofthe radio wave emitted to said first terminal for a predetermined periodincludes the step (S304) of lowering the intensity of the radio waveemitted to said first terminal by a level corresponding to a receivinglevel of a radio wave emitted from said first terminal when said secondterminal requests the connection.
 12. The antenna directivity controlmethod according to the claim 8, wherein said step of lowering theintensity of the radio wave emitted to said first terminal for apredetermined period includes the step (S408) of lowering the intensityof the radio wave emitted to said first terminal by a levelcorresponding to a receiving level of a radio wave emitted from saidfirst terminal in response to detection of the fact that a receivinglevel of a radio wave emitted from said second terminal is larger than apredetermined level when said second terminal requests the connection.13. The antenna directivity control method according to the claim 8,wherein said step of lowering the intensity of the radio wave emitted tosaid first terminal for a predetermined period includes the step (S508)of lowering the intensity of the radio wave emitted to said firstterminal by a level corresponding to receiving levels of radio wavesemitted from said first and second terminals in response to detection ofthe fact that the receiving level of radio wave emitted from said secondterminal is larger than a predetermined level when said second terminalrequests the connection.
 14. The antenna directivity control methodaccording to the claim 8, wherein said step of lowering the intensity ofthe radio wave emitted to said first terminal for a predetermined periodincludes the step (S12′) of directing a null direction of the radio waveemitted to said first terminal toward said second terminal when saidsecond terminal requests the connection.